It is well known in the prior art to utilize a laser within a facsimile or display system that may be used in turn for either recording, displaying or printing of alphanumeric text or pictures having varying shades of grey. In the prior art, an acousto-optic modulator is used to deflect a laser beam into a first order beam from its zero order beam position. That is, as a laser beam enters the crystalline structure of an unenergized acousto-optic modulator parallel plate crystal, at an angle thereto, the beam will be bent slightly at the point of entrance due to refraction but will continue through the crystal on a straight path to exit the far side of the crystal at an angle which is equal to the angle of incidence but on the opposite side of a horizontal center line through the modulator. This beam is called the zero order beam. When the acousto-optic modulator is energized and the laser beam is directed into the crystal at the bragg angle, the modulator crystal deflects the laser beam causing it to exit the modulator at an angle of diffraction which is equal to the angle of incidence and on the same side of the horizontal center line. This beam is the first order beam. For a more complete description of the operation of an acousto-optic modulator, attention is directed to application notes entitled "All About Bragg Angle Errors In Acousto-Optic Modulators & Deflectors", published September 1977 by ISOMET Corp., Springfield, Va. as publication AN772A.
In the prior art, the use of a stop to block the zero order beam while permitting the passage of the first order beam is well known. The difference between the exit angle of the beam or the angle of diffraction of a zero order beam and a first order beam is relatively small, on the order of 7 to 15 milliradians. Remember that one degree is equal to approximately 17 milliradians. Thus, to obtain a usable separation between the zero order and first order beam, it is necessary to separate the aperture stop from the acousto-optic modulator by a considerable distance, on the order of 10 to 15 inches. Further, in practice the acousto-optic modulator may not cleanly divide the incoming laser beam but may produce second order and third order beams as well as negative first and second order beams. The problem of plural random beams is further increased when, as in the present invention, multiple frequencies are applied to the acousto-optic modulator to develop a plurality of first order desirable beams.